1. Field of the Invention
The present invention relates generally to a shaft or other object motion initiator, and, more particularly, to such an initiator that is reliable and does not rely upon explosive means for operation.
2. Description of Related Art
There are many situations (e.g., fire fighting systems) in which it is necessary to provide motion of an object or a shaft with precisely timed initiation of the motion and in a reliable manner. In the past, one approach has been to spring-load a member to be moved while at the same time restraining it and explosive means were ignited to sever or rupture the restraining means effecting movement of the member by the spring action. The use of explosive means, however, is prohibitive under many circumstances.
Another approach is that taken in U.S. Pat. No. 3,163,732, Electrically Fused Spring Package, assigned to the same assignee as the present application, in which a coil spring continuously urges bodies 11 and 12 away from one another. An electrically fusible wire 19 extends between the bodies 11 and 12 preventing them from being separated by the spring. Application of a sufficient amount of electric current through the wire causes it to fuse allowing the spring to separate the two bodies from one another.
Still another approach is disclosed in U.S. Pat. No. 3,924,688, Fire Fighting System, assigned to the same assignee as the present application, in which two members 11 and 12 are held together by a wire 17 which is wrapped helically around the members and terminates in a hook-like end. A so-called hot wire 13 has a loop which engages the hooked end of the wire 17 holding the helically wound wire in place about the members 11 and 12, and in that way, securing the members together. When the wire 13 is heated by an electric current to a sufficient condition that it loses its tensile strength, it breaks. This allows the wire 17 to partially unwind from the members 11 and 12 permitting them to be separated by a spring-loaded plunger.
Although the latter patented device has been found to work satisfactorily for many applications, problems have been encountered during use. For example, where the current-carrying wire directly contacts the mechanically securing wire, over time and as a result of vibration, for example, a short circuit can result producing undesirable results. Also, on occasion, the heat produced in the wire by the electric current, as a result of the contact with the mechanically restraining wire as well as other parts of the apparatus, is conducted away from the hot wire to the extent that the hot wire does not lose its tensile strength and thus the apparatus does not operate to release the parts 11 and 12.